Flanged terminal pins for DC/DC converters

ABSTRACT

A dc/dc converter is mounted to a printed circuit board with rigid terminal pins which extend into a converter substrate to provide electrical connection to circuitry on the substrate. A terminal pin includes a flange which abuts the printed circuit board and spaces the converter substrate from the printed circuit board. Connection to the printed circuit board is made by solder provided between the flange and the circuit board.

RELATED APPLICATIONS

This application is a Continuation-in-Part of application Ser. No.09/740,707, filed on Dec. 19, 2000, now U.S. Pat. No. 6,545,890, whichclaims the benefit of U.S. Provisional Application No. 60/172,882, filedon Dec. 20, 1999. The entire teachings of the above applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Designers are increasingly using distributed power supply architecturesfor large electronic equipment. With this type-of architecture,electrical power is bussed throughout the equipment at a relatively highdc voltage, such as 48 volts. dc/dc converters mounted near the load(and often on the same printed circuit board as the load) then step thishigh voltage down to the low voltage required by the load (e.g. 3.3V),typically through an isolating transformer.

These point-of-load dc/dc converters typically have a low height (e.g.0.5″) so that the designer can place adjacent load boards close togetherin a card rack. The plan-view size of the converter must also be assmall as possible to leave more room on the load board for the loadcircuitry. Several standard sizes of converters exist, such as the “FullBrick” (2.4″×4.6″), the “Half-Brick” (2.4″×2.3″), and the“Quarter-Brick” (2.4″×1.45″). Other standard and non-standard sizesexist, as well. In general, the larger a dc/dc converter, the more powerit can handle.

Typically, dc/dc converters have a flat bottom surface formed by eithera housing or potting material. Terminal pins extend from this surface sothat the dc/dc converters can be “through-hole mounted” on a printedcircuit board (the “PCB”). When the converter's “through-hole pins” areinserted into the PCB's holes, the bottom surface of the converter makescontact with the PCB to ensure its proper positioning in the z-axisdirection.

Recently, “open frame” converters have been developed without a housingor potting. To achieve proper z-axis positioning, these converters useplastic or metal “standoffs” that keep the PCB and the converter'ssubstrate separated by a specified distance. Because these standoffseither abut or are attached to the converter's substrate, they take upspace on the substrate that could otherwise be used for electroniccomponents. They also partially or totally block the cooling air fromflowing under the open frame converter. Finally, the standoff representsan additional cost for the part and for its attachment to the converter.

Most electronic equipment manufactured today uses Surface MountTechnology (SMT) to attach their components to both the top and bottomsurfaces of a PCB. In this process, solder paste is first screen-printedonto the PCB in the locations of the component pads. The components arethen placed onto the solder paste. Finally, the PCB is passed through areflow oven in which the solder paste melts and then solidifies duringthe cool-down stage.

In comparison, dc/dc converters, with their through-hole pins, areattached to the PCB by either manual soldering or by an automatedproduction process called “wave soldering”. With this latter process,the PCB is first preheated and then passed over a molten pool of solder.The solder comes in contact with the bottom of the PCB, and it wicksinto the through-holes and solidifies after the PCB leaves the pool ofsolder.

A typical manufacturing process that requires both SMT and wavesoldering would first attach the SMT parts on the PCB, then insert thethrough-hole components, and finally pass the PCB through the wavesoldering machine. This process requires that the SMT components mountedon the bottom side of the PCB pass through a molten pool of solder.

As the distance between the leads on SMT packages gets smaller, itbecomes more difficult to pass these packages through a wave solderprocess and not have solder bridges form between adjacent leads.Furthermore, the heating associated with the wave soldering processcompromises the quality of the SMT components and their attachments tothe PCB. Manufacturers of electronic equipment are therefore interestedin avoiding the use of wave soldering altogether. Often, the dc/dcconverter is the only component on their boards that requires wavesoldering.

In response to this desire, several power supply manufacturers havecreated dc/dc converters designed to be surface mounted to a PCB.Instead of a few, large diameter through-hole pins, some of theseconverters have many smaller leads designed for surface mounting. Ingeneral, these surface mount pins make a dc/dc converter's overallfootprint larger than it might otherwise be since the pins typicallyextend beyond the converter's original footprint. Alternatively, atleast one manufacturer has introduced a product that uses a surfacemountable ball-grid. In this product, each through-hole pin of astandard converter is replaced with a conductive ball of sufficientdiameter to permit the converter to be attached to the PCB with SMTtechniques.

One important problem with all of these approaches for making a surfacemountable dc/dc converter is the relative weakness of a surface mountjoint compared to a through-hole pin. This problem is particularlyimportant since dc/dc converters have a higher mass than mostcomponents, and the mounting joints are therefore more susceptible toshock and vibration stresses.

Another problem with a surface mountable dc/dc converter is that theconverter's pins make electrical contact with only the outer conductivelayer in the PCB. Normally, the PCB's power and ground planes use innerconductive layers. With a surface mount connection, additional vias(that take up space and add resistance) are therefore required toconnect the outer conductive layer to the inner ones.

In comparison, a through-hole mounting is much stronger mechanically. Italso provides direct electrical attachment of the pin to the innerconductor layers of the PCB.

What is needed is a way to solder a through-hole mounted pin with areflow solder process, instead of using manual or wave soldering.

SUMMARY OF THE INVENTION

To address the problems mentioned above, a new through-hole terminal pinis used for mounting dc/dc converters or other circuit modules. In oneembodiment, this pin is similar to a standard through-hole pin, but ithas a circular flange near its bottom end. The diameter of the flange isgreater than the diameter of the PCB hole through which the lowerportion of the pin is inserted. The bottom of the flange therefore restsagainst the PCB's surface. It is located a specified distance from thedc/dc converter's substrate so that it provides the function of astand-off, but without taking up space on the substrate or requiring aseparate part. In addition, its interference with the cooling airflowunderneath the dc/dc converter is minimal.

In another embodiment, the through-hole pin has a flange near or at thetop end of the pin where it makes contact with the dc/dc converter'ssubstrate. The top of this flange rests against the bottom of thesubstrate. This arrangement improves the mechanical connection of thepin to the dc/dc converter's substrate, and it provides one way toensure the proper z-axis placement of the pin relative to the substrate.

In a third embodiment, the through-hole pin has one continuous, largerdiameter portion that performs the function of separate flanges oneither end.

In a fourth embodiment, the end of the pin has a cross-sectional shapethat is pointed along its periphery. This pointed shape facilitatespress fitting, or swaging, the pin into a hole of either the substrate,the PCB, or both. The press fit holds the pin in place for latersoldering in a hand, wave, or reflow process, and it improves themechanical strength between the pin and the substrate or PCB.

In addition, a process has been invented to permit this new through-holepin to be soldered to the PCB with a reflow process, instead of usingmanual or wave soldering. In one embodiment, this process works asfollows.

First, the pad around the PCB's through-hole is designed to becommensurate in size and shape with the flange of the converter pin.

Second, solder paste is screen-printed onto the PCB in the locations ofthe pads for both the SMT components and the dc/dc converter pins.

Third, both the SMT components and the dc/dc converter are placed on thePCB. The dc/dc converter, since it is relatively large and heavy, mightbe placed manually or by a special machine, although it could be placedby the same machine as the other SMT components. At this point, thebottoms of the flanges sit on top of solder paste, while the lower partsof the through-hole pins are inserted into their PCB holes.

Finally, the PCB is passed through a reflow oven in which the solderpaste first melts and then solidifies. During this step, the solderpaste between each pin flange and the PCB wicks down into thecorresponding PCB hole. The final solder joint between the pin and thePCB will therefore exist both underneath the flange and inside the PCBhole. With a properly designed pad and screening process, there willalso be a fillet of solder around the outer edge of the flange toprovide additional mechanical stress relief. The result is a very strongmechanical connection between the pin and the PCB, as well as a lowresistance electrical connection between the pin and both the inner andouter conductive layers of the PCB.

The flange facilitates this special soldering process. It provides aregion in which the solder paste directly contacts both the pin and thePCB. As the solder melts, it readily wicks along the surface of theflange and down the pin such that it fills the gap between the pin andthe PCB hole's via metalization.

In another embodiment of the reflow soldering process, the bottom end ofthe through-hole pin is given a cross-sectional shape that is pointed.When the pin is press fit into the PCB, the points of the pin hold thepin, and therefore the converter, in place. Solder is then applied tothe bottom side of the PCB in the region of the hole and its pad. ThePCB is then passed through a reflow oven in which the solder pastemelts, flows into the gaps between the pin and the hole, and thensolidifies.

In this alternative reflow soldering process the end of the inserted pinshould not extend beyond the bottom of the PCB. Otherwise, it mightinterfere with the solder application step. In fact, it is useful forthe end of the inserted pin not to reach the bottom side of the PCB(i.e., for the end to be inside the PCB). Such an alignment gives asmall “well” in the hole area, which increases the amount of solder thatcan be applied in this area. A flange near the bottom end of the pinfacilitates the correct insertion depth of the pin into the PCB hole,although there are other well-known means for controlling this depth.

This alternative reflow soldering process can also be used to attach thepin to the dc/dc converter's substrate during the construction of theconverter.

Thus, in accordance with one aspect of the invention, a dc/dc convertercomprises a converter substrate having circuitry thereon. At least onerigid terminal pin directly attaches to the converter substrate and iselectrically connected to the circuitry. The terminal pin includes aflange having a shoulder to abut a printed circuit board into which thepin is inserted and to which electrical connection is made. The shouldermay abut the printed circuit board by making direct contact thereto, orthrough one or more layers of material, such as solder. The shoulder isspaced from the converter substrate to accommodate spacing of theconverter substrate from the printed circuit board. Plural pins maytogether provide the spacing between the converter substrate and theprinted circuit board or one or more pins may operate with moreconventional standoff mechanisms.

To allow for a subsequent soldering process to, for example, solder theterminal pin to the printed circuit board, the components, materials andsolder connections of the converter may be such that they are notadversely affected by a 210° soldering process. In particular, thesolder used on the converter substrate has a melting temperature greaterthan 210° C.

The terminal pin may have a second shoulder which abuts the convertersubstrate. For example, the second shoulder may be on a second flangewith the pin extending from the second flange into the convertersubstrate. The second flange may be spaced from the first flange.Alternatively, a single flange may extend along a length of a terminalpin to abut both the printed circuit board and the converter substrate.In one embodiment, the flange has a uniform diameter.

The terminal pin may be swage fit into the converter substrate. To thatend, the pin may have a pointed cross-section shape. The portion of theterminal pin extending into the converter substrate may also be solderedto the converter substrate such as by a reflow soldering process.

The invention is particularly suited to a converter substrate havingcircuitry thereon in an open frame construction without a baseplatewhere the converter substrate is positioned parallel to the printedcircuit board.

In accordance with another aspect of the invention, a dc/dc powerconverter is mounted to a printed circuit board by soldering theconverter to the printed circuit board with the terminal pin extendingthrough a circuit board hole and the shoulder of the terminal pinabutting the circuit board to accommodate spacing of the dc/dc converterfrom the circuit board. Preferably, the solder is applied to the circuitboard or shoulder, and the shoulder is thereafter positioned to abut theprinted circuit board through the solder. The solder may be applied as asolder paste about the circuit board hole, and the hole may be leftsubstantially free of solder paste when the paste is applied. Theassembly may thereafter be subjected to a solder reflow process.

The solder may flow to form a fillet. For example, the solder may flowradially to form a fillet about the flange. The solder may also flowthrough the hole in the printed circuit board to form a fillet about aportion of the terminal pin exposed beyond the circuit board hole.

Solder may be applied to the holes from an opposite board side of theprinted circuit board after insertion of the terminal pins into theholes. Specifically, the solder may be applied from a molten pool ofsolder positioned below the printed circuit board.

In accordance with another aspect of the invention, solder is preappliedon the shoulder of the flange. For example, the solder on the flange maybe in a paste, may be a preform, or may be coated on the shoulder of theflange.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates a typical dc/dc converter with a housing or pottingand through-hole pins.

FIG. 2 illustrates an open-frame dc/dc converter (having no housing orpotting) that displays one example of a standoff structure.

FIG. 3 illustrates an open-frame dc/dc converter without a metalbaseplate that displays another example of a standoff structure.

FIG. 4 illustrates a through-hole pin with a flange.

FIGS. 5 a-c each illustrate a through-hole pin with a flange and an endwith a cross-sectional shape that is pointed.

FIG. 6 illustrates a through-hole pin with two flanges.

FIG. 7 illustrates a through-hole pin with a single flange that sitsagainst both the PCB and the substrate.

FIG. 8 illustrates a through-hole pin with two flanges where the topflange is flush with the top end of the pin.

FIGS. 9 a-e illustrate using a reflow process to solder a flangedthrough-hole pin to a PCB or a substrate.

FIGS. 10 a-c illustrate using a reflow process to solder a press-fitthrough-hole pin (having a cross-sectional pointed shape at its end) toa PCB or a substrate.

FIG. 11 is a cross-sectional view of a converter module mounted to aprinted circuit board in accordance with the invention.

FIGS. 12A-12B illustrate a pin having a chamfered flange.

FIGS. 13A-13C illustrates the flanged pin in a wave soldering process.

FIGS. 14A-14B illustrate an alternative flange in which the sides of theflange are completely shaved.

FIGS. 15A-15B illustrate a chamfered flange pin in a vibratory feeder.

FIGS. 16A-16C show a flanged pin in a reflow solder process.

FIG. 17 illustrates a solder joint resulting from a reflow process.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

Throughout this discussion and in the figures we will assume thecross-section of the pin and its flange is circular. One skilled in theart would know how to incorporate the concepts presented here for othercross-sectional shapes, such as triangular or rectangular.

FIG. 1 shows a typical dc/dc converter 100 with a metal baseplate 101(to which a heatsink might be attached), a housing or potting 102(inside which is the converter's circuitry), and its through-hole pins103. The pins have various diameters (e.g. 40, 60, and 80 mils) tohandle their rated current, and various lengths below the housing (e.g.110, 145, and 180 mils) to pass all the way through the PCB holes.

FIG. 2 shows an open-frame dc/dc converter 200 with a metal baseplate201 and through-hole pins 203. Since there is no housing or potting inthis converter, the converter's circuitry 202 is visible. In someopen-frame converters, the circuitry is mounted on a single substrate,and in other converters, two substrates are used. In either case, onesubstrate (the “baseplate substrate”) is either part of, or attached to,the metal baseplate so that the heat dissipated by the power componentson this substrate can readily flow to the baseplate.

FIG. 2 also shows a typical standoff structure 204 that is used on theconverter with no housing or potting. Standoff 204 is typically made ofplastic and is designed to abut the baseplate substrate. As can be seenfrom this figure, the standoff requires clear space (free of components)on the baseplate substrate. It also reduces the available space forother substrates and their components in the dc/dc converter.

FIG. 3 shows another open-frame dc/dc converter 300 that does not have ametal baseplate. Standoffs 301 are mounted on this converter's singlesubstrate 302, along with the converter's circuitry. The space thesestandoffs take is not available to circuit components. Through-hole pins303 are attached to substrate 302 using either a through-hole or asurface mount technique.

FIG. 4 shows a new through-hole pin 400 for dc/dc converters. This pinhas a shank 401 (in this case circular with a typical diameter of 80mils) and it has a flange 402 located along the length of the shank. Asshown in this embodiment, the flange is circular with a diameter and athickness that may, for example, be 120 mils and 40 mils, respectively.The flange diameter is larger than the diameter of the hole in the PCBso that when the bottom portion 406 of the pin (between 403 and 404) isinserted into the hole, the bottom side of the flange makes contact withthe top of the PCB.

The bottom side 403 of the flange is located a specific distance fromthe bottom end 404 of the pin. The length of portion 406 is chosen suchthat the bottom end of the pin will pass all of the way through the PCB.Typical lengths for 406 are 110 mils, 145 mils, and 180 mils, eachchosen to accommodate a different thickness PCB.

Through-hole pin 400 has its top end designed to be through-hole mountedto the dc/dc converter's substrate, as well. The length of the topportion 407 of the pin (between 403 and 405) and the depth to whichportion 407 is inserted into the hole of the converter substrate arechosen such that the bottom side 403 of the flange is located a specificdistance from the substrate. By doing this, the bottom side of theflange will hold the dc/dc converter substrate this specified distanceabove the PCB, thereby performing the function of a standoff.

The ends 404 and 405 of the pin can have various shapes, such as conicalor spherical, to facilitate the manufacture of the pin and the insertionof the pin into its mounting holes.

The top portion 407 of the pin may have design features that facilitateits mounting to the substrate. For instance, the pin might be pressfitted (or swaged) into the substrate's hole to hold it in place untilit is soldered and to provide a greater mechanical strength even afterit is soldered. If the cross-sectional design of 407 is circular,however, it would make contact with the side of the substrate holearound the entire perimeter. This tight fitting would not allow solderto wick down between the pin and the hole to provide a reliableelectrical connection between the pin and the inner conductor layers ofthe substrate.

FIGS. 5 a and 5 b show an alternative cross-sectional design for theupper portion 501 of portion 407 of the pin. The points 502 of thehexagonal design for 501 allow the pin to be press fit into thesubstrate hole while still leaving spaces 503 for the solder to wickdown into the hole. Other “pointed cross-sectional shapes,” shapes whichleave open space about the periphery between the pins and the side wallof the hole, such as other polygons or star-shapes, could accomplish thesame function.

Similarly, part or the entire bottom portion 406 of the pin could begiven a pointed shape so the dc/dc converter pin could be press fit intothe PCB and then soldered. FIG. 5 c shows an example 504 of such a pindesign.

One way to manufacture a pin with a cross-sectional pointed shape at itstop and/or bottom end is to start with a shank of the desiredcross-sectional shape. Another way is coin, stamp, impact-extrude, orturn on a screw machine to give an end of the pin its desired pointedshape.

To facilitate the mounting of the pin to the dc/dc converter'ssubstrate, the pin could have another flange near the top end of thepin, as FIG. 6 shows. The pin could be inserted or press fit into thesubstrate hole until the topside 603 of the top flange 602 makes contactwith the substrate. Flange 602 would thereby ensure that the pin isinserted (or press fit) the correct distance into the substrate hole. Italso provides additional mechanical strength to the connection betweenthe pin and the substrate, as well as additional electrical connectionbetween the two.

Another variation to the pin is shown in FIG. 7. In this figure, thefunctions of both the bottom flange 402 and the top flange 602 areaccomplished with a single flange 702. The standoff distance requiredbetween the substrate and the PCB determines the length of flange 702.This single-flange pin 700 can be easier to manufacture, have greatermechanical strength, and lower electrical and thermal resistance than atwo-flange pin design.

Another variation to the pin is shown in FIG. 8. In this embodiment, theconnection between the pin and the dc/dc converter substrate uses asurface mount, rather than a through-hole, technique. Flange 602 is nowflush with the top end of the pin. As such, its top surface, 801,provides a flat surface that can be soldered to the substrate with anSMT process. In another embodiment, by combining the concepts depictedin FIGS. 7 and 8, the pin would use a single flange with the top of theflange now flush with the top end of the pin.

The new through-hole pin described above can be wave- or hand-solderedto the PCB. It can also be reflow-soldered to the PCB with a processsimilar to that used for SMT components. As such, the new pin combinesthe mechanical and electrical advantages of a through-hole pin with theconvenience and compatibility of an SMT pin.

The method by which the new pin can be reflow-soldered to the PCB is asfollows.

First, as shown in FIG. 9 a, a pad 901 of exposed conductor around thehole 902 in the PCB 909 is made slightly larger in diameter than thediameter of the flange 903. Second, as shown in FIG. 9 b, solder paste904 is applied to pad 901. Third, the dc/dc converter is placed on thePCB such that pin 900 is inserted into hole 902 until the bottom offlange 903 rests on the solder paste 904, as shown in FIG. 9 c. The PCBand dc/dc converter are then passed through a reflow oven, which raisesthe temperature of everything until the solder past melts. Once melted,the solder wicks both down into the hole 902 and up the side of theflange 903. Finally, the solder is allowed to cool and solidify. Theresult, shown in FIG. 9 d, is a solder joint (or connection) 905 betweenthe pin and the PCB that exists within the hole, underneath the flangeand along the side of the flange. The “fillet region” 906 of the solderalong the side of the flange provides additional mechanical strength tothe solder connection and provides visual assurance that the solder hasfilled the region between the flange and pad. For best performance, thefillet should have a concave shape, as shown in the figure. Similarly,there should be a fillet 907 where the pin protrudes through the PCB.

A typical way to apply the solder paste 904 to the pad 901 is toscreen-print it onto the pad at the same time that solder paste isscreen-printed onto the pads for the PCB's other SMT components.However, pad 901 has a hole in the center of it and it is preferable tonot screen-print solder paste over this hole. FIG. 9 e shows one way toconfigure the opening 907 in the screen-printing stencil 910 to achievethis result.

It is important to apply sufficient solder paste to the pad 901 so thatthe solder connection will be electrically and mechanically sound. It isalso important to avoid too much paste, although this condition isgenerally less of a problem.

Depending on the size of the dc/dc converter's pin and its PCB hole, theamount of paste desired may be more than the amount applied by ascreen-printing process that works for the other, SMT components on thePCB. One way to get additional solder paste on pad 901 is to “overprint”the solder paste. With this approach, the opening 907 in thescreen-printing stencil 910 is larger in diameter than the pad 901. Thesolder paste printed outside the pad area initially sits on top of thesolder mask 908. During the reflow process, as this solder paste meltsit is drawn off the solder mask and into the desired solder joint regionby the action of surface tension.

Another way to apply the correct amount of solder paste on pad 901 is todispense it through a needle, rather than screen-print it. Thisdispensing process could be either manual or automatic.

A third way to apply the correct amount of solder on pad 901 is to use a“solder preform”, which is a thin sheet of solidified solder that hasthe desired shape and thickness and total volume of solder. This preformcan be applied to pad 901 with either a manual or automatic process.

Another way to apply the solder is to preapply it directly to theshoulder of the flange before the flange is positioned against the pad.For example, the solder could be coated on the underside of the flange,could be applied as a paste, or could be applied as a solder preform.The solder could be preapplied by the final installer or could bepreapplied by the pin or converter manufacturer.

With both the dispensing and the pre-form approaches for applyingsolder, it is again possible for the solder to extend initially beyondthe pad 901 and to sit on top of the solder mask 908. As with theoverprinting approach, the solder will be drawn off the solder mask andinto the solder joint region by surface tension during the reflowprocess.

Some experimentation will be required to determine how much solder pasteshould be applied in a given situation. The amount will depend on issuessuch as which solder application method is chosen, the size of the pin,its flange, and the hole, the thickness of the PCB, the number andthickness of the conductors in the PCB, the details of the reflowprocess, etc. An SMT process engineer of ordinary skill in the art wouldgenerally be able to determine a good starting point for thisexperimentation. Then, by mechanically inspecting the resultant solderconnection between the pin and the PCB, the engineer could easilydetermine whether the amount of solder used was too little or too much.In this manner, a final solution could be found after just a fewiterations.

As an example of how much solder might be used, consider the following:

-   1) shank diameter=80 mils-   2) flange diameter=120 mils-   3) flange thickness=40 mils-   4) hole diameter=90 mils-   5) pad diameter=160 mils-   6) PCB thickness=90 mils-   7) 6 layers of 4 oz. and 2 layers of 2 oz. copper within the PCB-   8) reflow process: 5 min ramp-up time, 210° C. peak temp for 1 min,    2 min ramp-down time

For this situation it has been determined that a solder volume of 106cubic mils gave a good solder connection.

Because the dc/dc converter and its pins usually have a higher thermalmass than other components on the PCB, the ramp-up time and theramp-down times in the reflow oven might need to be increased over thevalues used if the dc/dc converter were not present.

Since the dc/dc converter will be passed through a reflow oven, it isimportant to ensure that the converter's components, materials, andsolder connections are not adversely affected during this process. Forinstance, the converter might be fabricated with higher temperaturesolder than the one used to attach the converter to the PCB. A PCBsubstrate within the converter might have a higher temperature rating(e.g. 150° C. or 185° C.) instead of the normal 130° C. rating.

A typical solder which would be used to join the terminal pin to the PCBhas a melting temperature of 183° C. Thus, the conditions of the reflowoven are such that a peak temperature of the solder reaches about 210°C. as noted above at point 8. In order to assure the integrity of thedc/dc converter it is preferred that the solder used in the converterhave a melting temperature greater than 210° C. Preferably, a solderhaving a melting temperature greater than 230° C. is used in theconverter assembly.

A second method by which the new pin can be reflow-soldered to the PCBis as follows.

First, as shown in FIG. 10 a, the bottom portion 1001 of the pin 1000has a pointed shape to its cross-section so that it can be press fitinto the PCB 1002.

Second, when the pin is inserted into the PCB hole, the depth of theinsertion is controlled to keep the bottom end 1003 of the pin fromextending beyond the bottom surface 1004 of the PCB. Preferably, thebottom end of the pin should not reach the bottom PCB surface, butinstead remain slightly (e.g. 15 mils) inside the PCB, as shown in FIG.10 b. FIG. 10 b shows a flange 1005 near the bottom end of the pin thatfacilitates the correct insertion depth, although other means well knownto those skilled in the art could be used instead. For instance, amachine could be used to insert the pins, and the range of the machine'smotion could then be controlled to achieve the correct insertion depth.

Third, with the dc/dc converter held in place by the press-fit pin,solder paste can be screen printed onto the bottom side of the PCB, asshown in FIG. 10 c. Since the end 1003 of the pin does not extend beyondthe bottom surface 1004 of the PCB, the pin does not interfere with thisscreen printing. In addition, by leaving the end of the inserted pinslightly (e.g. 15 mils) inside the PCB, a slight “well” 1006 is formedin the area of the hole. During the screen printing process, this wellis filled with solder paste. The dimensions of the well can therefore beadjusted to achieve the desired amount of solder paste.

At this time, other SMT components can be placed on the bottom side ofthe PCB.

The PCB is then passed through a reflow oven where the solder melts,flows down into the gaps between the pin and the hole, and thensolidifies.

This same method can be used to solder the pin to the dc/dc converter'ssubstrate during the construction of the converter. The pin in thisfigure does not have a flange near the end of the pin that is insertedinto the substrate, although it might.

FIG. 11 shows how the final assembly might look in cross-section. Theopen-frame dc/dc converter has a substrate 1101 on which circuitry 1102is attached. Only a few circuit components are shown in this figure forsimplicity. In general, there would be many components mounted on bothsides of the converter substrate 1101.

Several terminal pins 1103 with flanges 1105 are swage fit into holes1106 of the converter substrate. These pins are then soldered to theholes in the spaces 1107 between the pins and the side walls of theholes. Conductive traces 1108 on the converter substrate 1101electrically connect the terminal pins to circuitry 1102.

The other end of pins 1103 are inserted into the printed circuit board 1104. The shoulder of the flange 1105 of these pins abuts the printedcircuit board.

Solder 1109 connects the pin 1103 and its flange 1105 to conductive padson the printed circuit board and the sidewalls of the holes. Fillets1110 of solder are formed around the flanges and around the end of thepin that extends through the printed circuit board.

The terminal pins 1103 are connected electrically to other parts of theprinted circuit board through conductive traces 1111.

Modifications to the flanges in the pins are shown in FIGS. 12-17.

As shown in FIGS. 12A and B, chamfers 1202, 1203 were cut on either sideof the pin flange 1201. The root of the chamfer is tangent to (1205,1206) the shank of the pin and extends upward at an angle 1204, in thiscase 45 Deg., though other angles could be used. The selection of thechamfer design was made so that the amount of remaining standoff area1207 could be maximized for proper support of the unit. The primarypurpose of these chamfers is to reduce the opportunity for voidformation during the process of soldering the pin into the end user'sprinted circuit board (EUPCB).

The typical process for soldering the pin into the EUPCB is wavesoldering. The converter is installed into the EUPCB manually or bymachine. It is then placed on the conveyor of the wave solder machine.The EUPCB moves through the several zones of the wave solder machine.Flux is applied to the bottom of the EUPCB using a spray or foam. Theflux may be a No Clean or Water Soluble formulation, and serves to coatthe solderable surfaces on the pin and EUPCB. Next the EUPCB travelsthough the preheat zone, where the flux is activated and breaks downsurface oxides on the solderable surfaces. The EUPCB then travelsthrough the waves of molten solder, typically two, one with laminar flowand one with turbulent flow. During this process some of the fluxcomponents evolve as gasses due to the magnitude and rapid change intemperature as the waves are crossed. The EUPCB then exits the wavesoldering machine and begins to cool.

FIG. 13A shows a cross-section of a pin 1301 seated in an EUPCB 1303during wave soldering. During the wave soldering process a situation canarise in which the rapidly evolving gasses 1305 might become trappedbeneath the flange 1302 on the pin 1301, given the rectangularcross-section without chamfers, in the following manner. The trappedgasses 1305, having no means of escape because the flange 1302 forms aseal 1306 at the interface between the flange 1302 and the copper barrelof the through hole 1304 on the EUPCB 1303, create a barrier for theadvancement of the liquid solder 1307 up the shank of the pin andprevent during the wave soldering process and prevents the solder 1307from flowing between the flange 1302 and the barrel of the through hole1304 to form a fillet 1308 around the flange 1302.

FIG. 13B shows how the new pin enables the escape of the flux gasses.The chamfers 1309 in the flange 1302 on two opposing sides allow anytrapped gasses 1305 to be vented in advance of the molten solder 1307 .As a result (FIG. 13C), a fillet 1308 can form on the top side of theplated through hole 1304 in the EUPCB 1303.

As shown in FIGS. 15A and B, the design of the chamfers does not affectthe ability for the pin 1500 to be fed automatically by vibratory bowlduring insertion. The top surface 1208 of the flange 1201 is used tostride the track 1501 of the vibratory feeder.

Other options for providing venting include shaving off the sides of theflange completely as in FIGS. 14A and B. Such options, however, createproblems for bowl feeding the pins 1400 because the ability to ride thefeed rails 1501 now becomes a function of orientation: When the axis1403 of this reduced flange 1401 is parallel to the rail 1501 slot, thepin can fall in and jam the bowl. The chamfers 1202, 1203 on the pins1200 do not intersect the top surface 1208 of the pin 1200, so that thetop side 1208 of the flange 1201 remains flat for riding the rails 1501.

The chamfers 1202, 1203 in the flanges 1201 also facilitate the FlangedPin in Paste (FPiP) reflow based process. Referring to FIG. 16A, whichshows the cross-section of the pin 1601 in the EUPCB 1603 during areflow solder process. Solder paste 1601 is deposited over the barrel ofthe plated through hole 1604. Per FIG. 16B, the pin 1600 is insertedinto the hole 1604 through the paste 1601 until it rests on the barrel1604. The paste deposit 1601 separates into two regions, the annularring 1607 left on the hole 1604 and a slug 1608 that remains on the tipof the pin 1605.

The openings 1602 between the flanges 1609 and the barrel 1604 of theEUPCB 1603 allow movement of the deposited solder paste 1607 from thetop side of the board through to the pin inside the barrel of the PTHduring reflow. Paste from the top surface overprint and the slug ofpaste 1608 that is carried on the tip of the pin 1609 migrate togetherduring reflow, being drawn by surface tension and wetting forces alongthe shank of the pin without impediment. This opening 1602 also allowsfor a wetting of the pin 1601 and barrel plated through hole 1604 of theEUPCB 1603 that is continuous as this opening is not cut off when thepin is fully seated.

The resultant solder joint 1701 is shown in FIG. 17, with fillets 1708formed about the chamfered regions (1709), wetting the entire hole 1704and shank of the pin 1706.

The design of the chamfers also leaves a large flat surface 1207 on thepin 1200 to bond with the annular ring of the plated through hole 1304on the EUPCB 1308 for a stronger surface joint on top of the PCB, andside fillets 1708 that form in the chamfered regions 1709, give extrasurface area for bonding. The amount of bonding surface parallel to thesurface of the EUPCB is an important factor in the distribution ofstresses at the interface of the pin and the solder. Stressdistributions are kept to a minimum and thereby decreasing the magnitudeof stains generated due to mechanical loads and thermal mismatch.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method of mounting a dc/dc power converter on a printed circuitboard comprising: providing a dc/dc converter comprising: a convertersubstrate having circuitry thereon; and at least one rigid terminal pinattached to the converter substrate, the pin being electricallyconnected to the circuitry, the terminal pin including a flange having ashoulder spaced from the converter substrate; applying solder to atleast one of the shoulder of the flange and the circuit board;positioning the dc/dc converter to the printed circuit board with theterminal pin extending through a circuit board hole and the shoulderabutting the circuit board to accommodate spacing of the dc/dc converterfrom the circuit board with solder between the shoulder of the flangeand the printed circuit board; and subjecting the dc/dc converter andprinted circuit board to a solder reflow process to join the terminalpin and printed circuit board with the solder.
 2. A method as claimed inclaim 1 wherein the dc/dc converter comprises a substrate with circuitrythereon in an open frame construction.
 3. A method as claimed in claim 1wherein the solder paste is applied as a paste about the circuit boardhole.
 4. A method as claimed in claim 3 wherein the hole is leftsubstantially free of solder paste when the paste is applied.
 5. Amethod as claimed in claim 1 wherein the solder flows within the hole ofthe printed circuit board.
 6. A method as claimed in claim 1 wherein thesolder flows to form a fillet.
 7. A method as claimed in claim 6 whereinthe solder flows radially to form a fillet about the flange.
 8. A methodas claimed in claim 6 wherein the solder flows through the hole in theprinted circuit board to form a fillet about a portion of the terminalpin exposed beyond the circuit board hole.
 9. A method as claimed inclaim 1 wherein the components, materials and solder connections of theconverter are not adversely affected by a 210° C. soldering process. 10.A method as claimed in claim 1 wherein solder used on the convertersubstrate has a melting temperature greater than 210° C.
 11. A method asclaimed in claim 1 further comprising a second flange on the terminalpins that abuts the converter substrate, and the pin extends from thesecond flange into the converter substrate.
 12. A method as claimed inclaim 1 wherein the flange extends along a length of the terminal pin toabut the converter substrate.
 13. A method as claimed in claim 1 whereinthe terminal pin comprises a second shoulder which abuts the convertersubstrate.
 14. A method as claimed in claim 13 wherein the terminal pinextends into the converter substrate.
 15. A method as claimed in claim14 wherein the terminal pin is swage fit into the converter substrate.16. A method as claimed in claim 15 wherein the portion of the terminalpin extending into the converter substrate has a pointed cross sectionalshape.
 17. A method as claimed in claim 16 wherein the portion of theterminal pin extending into the converter substrate is soldered to theconverter substrate.
 18. A method as claimed in claim 1 wherein theterminal pin extends into the converter substrate.
 19. A method asclaimed in claim 18 wherein the pin is swage fit into the convertersubstrate.
 20. A method as claimed in claim 19 wherein the portion ofthe pin extending into the converter substrate has a pointed crosssection shape.
 21. A method as claimed in claim 20 wherein the portionof the terminal pin extending into the converter substrate is solderedto the converter substrate.
 22. A method as claimed in claim 1 furthercomprising applying solder on the shoulder of the flange prior topositioning the terminal pin in the circuit board hole.
 23. A method asclaimed in claim 22 wherein the solder is applied in a paste.
 24. Amethod as claimed in claim 22 wherein the solder is applied as apreform.
 25. A method as claimed in claim 22 wherein the solder iscoated on the shoulder.